The use of filter dyes in photographic elements is well known. Filter dyes may be located in radiation-sensitive layers, in overcoat layers, in layers adjacent to radiation-sensitive layers, in interlayers in multilayer elements, in undercoat and antihalation layers, and on the side of the support opposite the radiation sensitive layer. When incorporated in the radiation sensitive layer, filter dyes can improve sharpness by absorbing light scattered from one silver halide grain to another. Filter dyes can also be used in antihalation layers to retard the sensitivity of one light sensitive layer to another in a multilayer element by absorbing unwanted radiation due to reflection or refraction.
In these as well as the many other uses for filter dyes in photographic elements described, for example, in U.S. Pat. No. 4,855,221, it is important that the dyes do not wander or diffuse into adjacent layers. Filter dyes must also be completely decolorized or removed from photographic elements, or both, usually during processing when their function is complete, or staining results. The latter problem is compounded when a polymeric mordant is used in the filter layer to prevent dye wandering.
Filter dye stability, particularly at high temperature and humidity conditions is also important. It is often also highly desirable that the filter dye has a steep absorption peak; that is, that the dye is "sharp cutting".
One method used to incorporate soluble filter dyes into photographic film element layers involves adding them as aqueous or alcohol solutions. Dyes introduced by this method often wander into other layers of the element, usually with deleterious effect. While the use of polymeric mordants can prevent dye wandering, such mordants aggravate the stain problem encountered when the dye remains in the element through processing.
Filter dyes have also been prepared as conventional dispersions in aqueous gelatin using standard colloid milling or homogenization methods or as loaded latexes. More recently, ball-milling, sand milling, media-milling, and related methods of producing fine particle size slurries and suspensions of filter dyes have become standard tools for producing slurries and dispersions that can readily be used in photographic melt formulations. Solid particulate filter dyes introduced as dispersions, when coated at sufficiently low pH, can eliminate problems associated with dye wandering. However, filter dye dispersions prepared by milling tend to have slow wash-out rates resulting in lengthy processing or unacceptable dye staining. Further, milled particulate filter dyes provide relatively low absorption coefficients, requiring that an excessive amount of dye be coated. Among other things, milled dyes can also provide too broad an absorption envelope for the contemplated use; the wavelength of maximum absorption may not be at optimal position in the visible region, or the long-wavelength absorption edge of the absorption band may not be optimally placed for the intended application.
In addition, the time and expense involved in preparing serviceable solid particulate filter dye dispersions by milling techniques are a deterrent to their use, especially in large volume applications.
Sodium salts of color couplers containing carboxy groups have been incorporated in photographic binders and elements in the pH range of 6 to 6.5 in order to disperse photographic dyes. However, such expedients can cause recrystallization effects resulting in unwanted sensitivity changes and degradation of gradation and color saturation.
The precipitation of hydrophobic photographically useful components such as couplers from solution to obtain small particle size dispersions is also known. Such processes generally involve dissolving the photographically useful component in a water miscible organic solvent, optionally augmented by the addition of base to ionize the component and increase solubility. Dispersing aids such as surfactants or steric stabilizers can be added with subsequent precipitation achieved by lowering the pH or by flooding the solution with water or other solvent in which the component is insoluble.
One such prior process disclosed in U.K. Patent 1,193,349 disperses color couplers that contain no sulfonic acid or carboxylic acid solubilizing groups from solution in a mixture of water-miscible organic solvent and aqueous alkali using a homogeneous acid solution to form a colloidal precipitate which is stabilized with dispersing aids. Large amounts of water-miscible organic solvent are used, greater than 50% of the total solvent mixture, as well as high concentrations of aqueous alkali. Such high concentrations of alkali cause severe decomposition of filter dyes. American Institute of Chemical Engineers Symposium Series (Vol. 76, No. 193, pp 43-51, 1980) discusses a continuous precipitation process for dispersing spherical particles of metallized dye of the type commonly used in instant photographic image-transfer systems. In the process described, the dyes are dissolved in a water-miscible organic solvent in the presence of dispersing aids and precipitated as stabilized colloidal dispersions when the organic homogeneous solution is diluted with water until the dye is no longer soluble in the solvent. The solvent is then removed to provide an aqueous dispersion of colloidal particles. Unfortunately, the process is not adaptable to large scale manufacturing operations, nor is it applicable to dyes that are insoluble in water-miscible organic solvents.
U.S. Pat. No. 4,388,403 teaches a process for preparing dispersions of hydrophobic photographic couplers in water using a variety of polymers and polymeric latexes to aid dissolution and/or stabilize homogeneous solutions and heterogeneous suspensions of the couplers in water-miscible organic solvents and mixtures thereof with water. A dispersion is obtained when the organic solvent is removed. This process can only be used with materials that are soluble in water-miscible organic solvents in applications that can tolerate the ionomeric polymers and latexes used as dissolution aids and stabilizers. It is not advantageous with components that will migrate out of the polymeric stabilizing aggregate or latex to form crystals in the stored dispersion or photographic element due to trace solubility in water.
U.S. Pat. No. 4,933,270 discloses a method for forming dispersions of hydrophobic photographic components having groups that are destroyed in base (high pH) solutions, particularly ester-terminated photographic couplers. U.S. Pat. Nos. 4,957,857 and 5,015,564 disclose processes for stabilizing precipitated dispersions of hydrophobic couplers and U.S. Pat. No. 5,008,179 discloses a process for the preparation of dispersions of photographic couplers and the use thereof in photographic elements having increased activity.
These and other prior processes require removal of organic solvent by activated stripping or distillation, evaporation, or extensive washing, and bring with them the attendant disadvantages of such operations. Generally, if elevated temperatures are not used, too little solvent will be removed. If elevated temperatures are used, extensive ripening and particle size growth often occurs while thermally unstable components decompose. If a vacuum is used to remove solvent, extensive foaming often results.
If all of the solvent is not completely removed, recrystallization (ripening) during storage of the dispersion or of the coated photographic element containing the dispersion can occur. Further, residual solvent can cause agglomeration and coalescence of particles in the hydrophilic-binder melt and flocculation of the dispersion in the coating melt.
Notwithstanding, dispersions of filter dyes particularly useful in photographic applications are not prepared by such methods. Generally, a milling or grinding procedure such as ball milling is used to prepare filter dye dispersions. Such processes are slow and cumbersome and do not provide uniformly very small particle sizes.
U.S. Pat. No. 4,855,221 discloses that oxonol filter dyes may be prepared and purified by dissolution with a base followed by reprecipitation in excess concentrated acid (pH&lt;1 in the slurry), such as concentrated hydrochloric acid. Such macroscopic precipitation processes yield agglomerated particles which are easy to wash and filter and are highly suitable for the bulk solid storage of said dyes. The agglomerates and slurries of the same are not small particle colloid dispersions, and such agglomerates are unsuitable for use as photographic filter dyes without extensive treatment by mechanical milling techniques (roller milling, sand milling, etc.) in the presence of dispersing aids and stabilizers.
A need therefore exists for a process for dispersing filter dyes that is faster than ordinary milling procedures and that provides solid particulate filter dyes that do not wander but wash out easily during processing leaving little or no residual stain. The dispersion process should provide average particle sizes as small as or smaller than can be obtained by conventional methods, such as grinding, and filter dye dispersions that have hues that differ from those obtained when grinding processes may be employed. Further, a filter dye dispersion process is needed that will provide dispersions having improved covering power and high light absorption coefficients with sharp-cutting absorbance peaks.